In some systems, a capacitive load may vary greatly. In general, the capacitive load may vary over time in a predictable or random manner. A voltage source which is referred to ground is typically utilized to supply a constant reference voltage to the varying capacitive load. As the capacitive load varies, the charge required by the voltage source will vary. Due to a high impedance output, the voltage source is current limited, and therefore cannot sink or source large amounts of charge in short periods of time. This current sinking limitation may cause a drop in the desired reference voltage across the capacitive load, which may yield undesired system behavior.
One method of combating this current limitation is the use of a buffer. Generally, the buffer has voltage feedback which lowers the output impedance and provides unity gain. This feedback configuration is capable of outputting a large amount of current when required. When the required capacitive charge varies, the buffer is able to supply sufficient current to the capacitors, whereas an un-buffered voltage source could not. The response of the buffer is an important concern, particularly because feedback may cause instability. In general, as the charge required of the capacitive load varies and the output of the buffer is driven away from the reference voltage, the output of the buffer should return to the reference voltage quickly without excessive ringing or overshooting. To obtain a stable response without ringing or overshooting, the buffer should have a phase margin that is sufficient for all charge conditions.
The present invention provides an improved load compensation system. Furthermore, as described below, the invention provides compensation to loads that vary in either a predictable or a random manner.